Some body frames of motorcycles include a head pipe to which a steering shaft with a steering handle attached to an upper portion thereof is rotatably mounted, a main frame extending rearward from the head pipe, a down tube extending downward from the head pipe, a lower frame extending substantially rearward from a lower end portion of the down tube, and a pivot frame (also referred to as a swing arm bracket) that extends downward from a rear end portion of the main frame and is coupled at a lower end portion thereof to a rear end portion of the lower frame. The main frame in some cases includes a pair of right and left frame members.
An engine is mounted into an inner space of the body frame defined by the main frame, the pivot frame, the down tube, and the lower frame by a number of brackets. A straddle seat is mounted over the main frame. Straddling the seat, a rider steers the motorcycle. A fuel tank is disposed above the main frame and in front of the seat. Since a load of the engine, the fuel tank, and a rider is applied to the body frame and a vertical impact or a torsional force is externally applied to the body frame during travel of the motorcycle, the body frame is required to have high stiffness. Since the head pipe of the body frame is more likely to be subjected to the load, the impact, and the external force, it is required to have sufficient stiffness. Furthermore, it is necessary to firmly couple the head pipe, the main frame, and the down tube to each other.
For example, Japanese Utility Model Application Publication No. Sho. 62-162182 and Japanese Laid-Open Patent Application Publication No. 2004-256073 disclose a head pipe that is manufactured by forging to have a shape that enables a head pipe, a main frame, and a down tube to be coupled firmly to each other. As shown in FIG. 8, a conventional head pipe 80 includes a tubular shaft support portion 81 to which a steering shaft is rotatably mounted, a main frame tongue portion 82 configured to extend substantially rearward from an upper portion of the shaft support portion 81, and a down tube tongue portion 83 configured to extend substantially downward from a lower portion of the shaft support portion 81, and is manufactured by forging.
A main frame 85 is welded to the head pipe 80 with an upper surface of a front portion thereof in contact with a lower surface of the main frame tongue portion 82 of the head pipe 80 and a front end portion thereof in contact with a rear surface of the shaft support portion 81 of the head pipe 80. A down tube 86 is welded to the head pipe 80 with a front surface of an upper portion thereof in contact with a rear surface of the down tube tongue portion 83 of the head pipe 80 and an upper end portion thereof in contact with a rear surface of the shaft support portion 81 of the head pipe 80. The head pipe 80, the main frame 85 and the down tube 86 are firmly coupled to each other with a sufficiently long weld length.
In the conventional head pipe 80 illustrated in FIG. 8, the main frame tongue portion 82 and the down tube tongue portion 83 are forged by using a die divided into right and left parts or front and rear parts. In this case, however, the main frame tongue portion 82 has an unwanted thick region (e.g., region 82A in FIG. 8), increasing the weight of the body frame. For this reason, a process for cutting the thick region 82A to reduce weight of the head pipe 80 becomes necessary.